Kai Ryssdal: Forty years ago this Friday, the phrase "War on Drugs" first passed presidential lips. Since the days of Richard Nixon, the United States has spent hundreds of billions of dollars trying to control the use of illegal drugs and keep them out of the country. It's been mostly a losing battle. Twenty-two million of us use illegal drugs every year. That demand gets a lot of blame for the never-ending supply that comes across the Mexican border.

Izzy Cano: All right, just go ahead and sign here, sir. And your initials on the side of the bottle.

Peter O'Dowd: At this drug screening center in Phoenix, a line of men goes one-by-one into the bathroom to pee into a plastic bottle. This center works with Arizona's criminal justice system to keep tabs on probation and parole violations. The lab tests several million samples a year. In many ways, the people here are America's typical users. Most are men. Marijuana is the drug of choice.

Barbara Zugor: This is the gut of life, isn't it?

Executive Director Barbara Zugor has worked here since 1977. Sometimes she's philosophical about her job. Why in 2009, for instance, did almost 17 million Americans smoke pot habitually? Why did a million and a half use cocaine? A survey of 17 countries by the World Health Origination shows Americans use these drugs more often than any other country in the study. But why?

Zugor: Now, that's the $64 billion question. And we're all trying to figure out why is that? And until we find that out, there will probably be drugs and drug abuse in this country.

Drug dealer: There's a ripple effect to everything.

Even drug dealers think about this.

Drug dealer: Everyone who smokes weed is like a crack addict. That's their fix. That's their drug. And they're going to call your phone.

I can't tell you his name -- for obvious reasons. But the demand for his product was constant. Before getting busted in Phoenix, he bought several pounds of Mexican pot at time. The weak stuff, Bobby Brown it's called. He sold it for 60 bucks an ounce, and made about $2,000 a week. He sold to students, to firemen, even to military police.

Drug dealer: I think that Mexicans are getting a bad rap. And while a lot of the stuff is coming across the border, it's our demand that's pushing it here. It's like stop the demand, you'll stop the flow.

Now, this is remarkable. The U.S. government echoes the sentiment of a drug dealer.

Gil Kerlikowske: Mexico wouldn't have quite the level of problems -- and actually several other countries wouldn't either -- if we could reduce our own demand.

A snapshot of drug use in the U.S. (Courtesy U.S. Department of Health and Human Services).

Gil Kerlikowske is the U.S. drug czar. He says the demand for cocaine has fallen steadily over the last few decades, but the government isn't sure why. It could be a combination of prevention efforts like the "Just Say No" campaign from the 1980s and 90s. Or it could be the billions of dollars invested in law enforcement on both sides of the border. For decades, there have been critics who say this so-called "War on Drugs" is entirely the wrong approach. And Kerlikowske agrees on one point: the phrase should be thrown out the window.

Kerlikowske: We should stop calling it a "War on Drugs." People are frustrated. They don't see a real end result or a win. They know that somebody, their son or daughter in high school, can still get drugs. And it's just the wrong approach.

Instead, Kerlikowske says this drug problem must also be treated as a human health issue. It won't be easy. Economists say very little can be done in the short term to stop our hunger for drugs like marijuana. Its use has remained steady for a decade.

Drug user: Whenever I hear about the violence, I cringe.

This is a Phoenix drug user, who spends a lot time thinking about this very debate. She started smoking marijuana after a cancer diagnosis. Despite Arizona's new medical marijuana law, she doesn't have permission to smoke. She buys it illegally.

Drug user: One of the guilt parts that I have over using the marijuana is what's going on in Mexico. I don't want to feel that what I'm having to do for my health is hurting somebody else.

And yet, the money she spends on marijuana is more than likely fueling the Mexican drug cartels. All those little drug sales, all over America, add up to as much as $39 billion each year that heads south across the border.